Conceptus Cavities Week 2 and Week 3

Outside

Continued expansion of syncitiotrophoblasts within the endometrial wall opens both uterine glands and uterine blood vessels. These spaces outside the conceptus fill with uterine gland secretions and maternal blood forming maternal blood-filled lacunae. These lacunae provide the initial nutrition to the growing conceptus, which will later be provided through the placenta.

Inside

A cavity forms now between the inner cell mass and the trophoblast (cytotrophoblasts) wall. This cavity is the amniotic cavity.

A new cell layer forms from cells proliferating and lining the inside of the blastoceol cytotrophoblastic layer (extraembryoic mesoderm). This extraembryoic mesoderm layer continues to proliferate and then vacuolates, splitting into two separate cavities.

Overview of blastocyst implantation in uterine wall during the second week of development. (Image: Moore & Persaud, 1998)

Human Conceptus Implantation 2nd and 3rd Week of Development

Identify secondary chorionic villi, maternal blood "spaces" (empty) and uterine tissue (maternal decidua). The embryo lies between the yolk sac fluid and the amniotic sac fluid. Note that the ends ("bases") of some long secondary chorionic villi are attached to the maternal (decidual) tissue by dense clusters of trophoblast cells (the cytotrophoblastic column) - these are anchoring secondary villi. Other shorter, secondary villi are "free-floating" in the maternal intervillous spaces, from which the blood has drained into the large endometrial veins. (Image: Nishimura etal., 1977)

File:Day16lp.jpgCarnegie Stage 6-7, human conceptus approximately 16-18 days. (about the time of the first missed menstrual period).

Human Conceptus (high power view)

Identify the three fluid-filled sacs, the embryo, and the body (connecting) stalk indicating the caudal end of the embryo. Note the thin amnion and the thicker wall of the yolk sac and conversely, the thick ectoderm and thin endoderm. Vascular channels in connecting stalk. The broad elevation in the ectoderm is the expansion dome (primitive single brain bulge) and the cluster of cells at the caudal end of the embryonic disc is the primitive node region. Note the precipitated protein in the extra-embryonic coelom and in the yolk sac, and the start of the formation of the intra-embryonic coelom between ectoderm and entoderm, at the cranial (rostral) end of the expansion dome.

Identify the extraembryonic mesoderm which surrounds the yolk and amniotic sacs, and continues to form the connecting stalk, where it then continues to line the chorionic sac. Extra- and intra-embryonic mesoderm merge along the perimeter of the embryonic disc.

At the top right of the micrograph is a secondary chorionic villus, lying in an empty maternal intervillus space. The villous has an outer layer of syncytiotrophoblast (smaller, darker nuclei), an intermediate layer of cytotrophoblast and a core of mesoblast (extra-embryonic mesoderm).

Terms

blastocyst- the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the conceptus to adplant and then implant into the uterine wall.

inner cell mass- the clump of cells found inside the blastocyst. These cells will go in to form the embryo, these are the "stem cells" (we here about in the media) that are totipotential, they can form any tissue in the embryo. Mature oocyte-the female germ cell released at ovulation from the ovary.

parental genomes- the male (sperm) and female (oocyte) DNA which contributes to the embryo's cells.

trilaminar embryonic disc- the 3 layered embryo stage.

trophoblasts- (Gr. trophe = nutrition) outer layer of cells on blastocyst that will generate the embryonic part of the placenta.